The present invention is an infeed conveyor for a saw that can optimally feed cants or logs having varying amounts of sweep from piece to piece. The invention further includes the combination of the conveyor with a circular saw or saws. This permits logs to be sawn around the curve of their sweep to obtain maximum lumber recovery.
Within a few more years, the last of the old growth timber available for harvesting in the United States will have been cut. Most of the sawmills which formerly depended on large old growth logs have already had to convert the equipment in their mills to that which is suitable for handling much smaller logs at high throughput volumes. One common characteristic of second growth timber is that the logs usually have varying but larger amounts of sweep or curvature. The presence of sweep results in yields which are significantly lower than those which are attainable from straight logs. This is because much of the curved portion must be machined off in order to produce cants or lumber having the usual configuration of a rectangular parallelepiped.
Many small log sawmills orient the log prior the primary breakdown saw so that the greatest curvature is either up or down (horns up or horns down) rather than side to side. These openings cuts may be made either by saws, which remove side boards and slabs, or by chippers which reduce the slabs to wood chips suitable for pulping. The result is a cant having parallel faces on two sides. As noted earlier, cants sawn in this fashion tend to have appreciable sweep. Sweep is here defined to mean the curvature on the concave edge of the cant when the cant is placed on one of its flat faces. While occasional instances of extreme sweep occur, in most cases sweep is rarely larger than about 100 mm in a cant approximately 5 m long (about 4 in. in 16 ft.). Most typically, it will average between 25 and 50 mm in 5 meters.
Man has ad to attempt to cope with geometric irregularities in logs ever since he began to utilize trees. Thus, it is not unexpected that the prior art shows previous attempts to deal with sweepy cants and to devise schemes for improving the yield from cants of this type.
As general background to the present invention, reference can be made to U.S. Pat. Nos. 259,661 to Bowker and 1,263,443 to Lien. Both of these inventors devised schemes for sawing wooden barrel hoops along a path that was precisely parallel to the outside surface of the log. This was deemed necessary in order to reduce the presence of cross grain which would reduce the strength of the hoop. Somewhat more pertinent is Great Britain Pat. No. 545 of 1852. In this sawing device, a tree was placed on a carriage which could be moved in a circular arc with respect to a saw. The purpose here appears to be to cut complex curved and/or angled ships timbers. However, it appears inherent in the invention that cuts could be made parallel to the surface of a curved log.
Japanese Patent No. 49-7557 shows a device used for "sawing around the curve" of a cant which contains sweep. The cant is first sawn to produce one good edge parallel to the original surface. After this point, additional boards can be taken off parallel to this surface by manually steering the cant as it emerges from the saw.
Swedish Patent No. 33,098 is a sawing device having feed rolls which can be canted so that the axes lie at an angle and intersect at a point corresponding to the arc of a curve along which a cut is to be made. U.S. Pat. No. 3,685,556 to VanSickle takes a somewhat different approach. This inventor shows a device for use with a shop band saw for cutting predetermined contoured pieces from straight stock to make; e.g., Christmas tree stands. More conventional approaches are described in U.S. Pat. Nos. 3,665,984 to Ackerfeldt, 4,416,312 to Ostberg and Swedish Patent No. 306,415. These, in general, deal with positioners for optimizing yields from curved cants by straight line sawing. The patnt to Ackerfeldt is valuable for the background it gives to this type of approach.
While it has been noted that increased yields can be obtained by "sawing around the curve," until very recently nothing has been done to make this a practical approach in a modern, high-speed sawmill. The closest examples can be found in U.S. Pat. Nos. 4,144,782 and 4,219,056 to Lindstrom and U.S. Pat. No. 4,373,563 to Kenyon. The earlier of the two Lindstrom patents is of particular interest. In the device described, a log is fed into what is preferably a single band saw along a path which follows the curvature of the log. The log lies on a roll case with two pairs of opposing vertical guide rollers upstream from the saw. These guide rollers embrace the cant as it is fed into the saw. They are mounted on frames transversely displaceable to the path of the log so that the log is directed along a curve to which the saw blade is tangent. Lindstrom further notes that the bowed boards resulting from around-the-curve sawing will generally flatten out when they are kiln dried. Kenyon have had limited commercial application although However, the cant is oriented by lateral pressure applied only ton ne side of the cant by a roller located upstram from the point of sawing.
To the present inventors' knowledge, the devices taught by Lindstrom and Kenyon have had limited commercial application although their use has not become widespread. This may possibly be because they are of limited use in a high-speed sawmill which may typically run 10-20 cants per minute through a gangsaw.
Practical means for sawing logs around their sweep curvature at the high speeds necessary in modern sawmills has recently been disclosed in the following patents: Hasenwinkle et al, U.S. Pat. No. 4,633,924; Wislocker et al, U.S. Pat. No. 4,653,560; and Hasenwinkle, U.S. Pat. No. 4,690,188. The first of these patents shows a line bar which can be bent to conform to the sweep curvature of an individual log or cant. This line bar is set up along the edge of a roll case or similar conveyor to guide the piece being sawed into a vertical gang saw. The second patent is similar, but here the line bar is of fixed curvature which approximates the average sweep curvature of the cants being sawn. While the first of these methods will give somewhat higher lumber recovery, it is not particularly efficient in the use of available space. For sawing logs that have considerable sweep, the infeed end of the line bar must often by deflected as much as 60 cm, or even more. Frequently there is not adequate room for deflection of this magnitude in a sawmill, particularly when the line bar is used to retrofit an existing saw. The apparatus described in U.S. Pat. No. 4,690,188 was then developed to overcome this difficulty. Here the line bar is bent to a configuration equivalent to the radius of curvature of the incoming cant. However, in this case the saws are also skewed somewhat, depending on the amount of line bar curvature. Using this method, line bar deflections of as little as 20 cm can accommodate even those cants having considerable sweep. This latter system can be used with either vertical or horizontal arbor gang saws. However, no wholly satisfactory log infeed system had been developed at the time of this patent for use with a vertical arbor saw. It was a requirement that the lower portion of the cant slide along the line bar and feeding had to be accomplished either by using a series of opposed drive rolls acting against each side of the cant or by using a series of single driven rolls working against the top of the cant. In either case, the inherent inaccuracy in the feeding system frequently resulted in lumber with poor dimensional accuracy. The present invention has overcome these problems and contributed many other desirable features which improve accuracy and yield in a high speed, small log sawmill.